Flight control systems are automation facilities making it possible, in their basic operating modes, to bring an aircraft into and maintain it in a flight configuration corresponding to a predetermined speed vector and/or trim vector, and, in so-called “higher” operating modes, to bring an aircraft onto a type of predefined trajectory. They act on the flight controls, either directly, when they operate as automatic pilot, or by way of the pilot whom they provide with instructions, when they operate as flight director. They incorporate computers programmed to estimate the deviations existing between the trim, the speed vector or the current trajectory of the aircraft with respect to a trim, a speed vector or an instruction trajectory and to deduce therefrom the orders to be executed by the flight controls or the direction and amplitude of the maneuvers to be advised to the pilot so as to reduce these deviations.
In order to imitate the behavior of a pilot as closely as possible, flight control systems are designed with two levels of control loops: a first level of control loops termed “large loop” determining the values that must be taken by the primary flight parameters used by pilots when flying by instruments to reach the correct altitude, speed vector or trajectory, and a second level of control loops termed “small loop” translating the parameter values delivered by the large loop, either into commands for the actuators controlled by the flight controls for automatic-pilot operation, or into indications of deviation to be corrected, displayed in the cockpit, on the PFD screen for viewing the primary flight parameters for flight-director operation.
With conventional flight controls, having simple mechanical, hydraulic and/or electrical assistance, comprising a stick allowing the pilot to prescribe the position of the airfoils and admitting the longitudinal attitude angle as primary control parameter in the vertical plane, flight control systems comprise, for controlling the trajectory parameters in their various operating modes, large loops delivering longitudinal attitude angle commands utilized by small loops delivering, during automatic-pilot operation, the signals for actuating the controls and, during flight-director operation, a longitudinal attitude instruction made available to the pilot on the PFD screen.
With electric flight controls (fly-by-wire) comprising a stick or lever allowing the pilot to enter, in relation to the vertical axis, instructions regarding load factor, which is likened hereinafter to a vertical acceleration although in all strictness, a cosine coefficient due to the angle of roll comes into the relation which links them, and therefore admitting the load factor as primary flight parameter instead of the longitudinal attitude angle, flight control systems comprise, for controlling the trajectory parameters in their various operating modes, large loops delivering load factor commands utilized by small loops delivering, during automatic-pilot operation, the signals for actuating the controls and, during flight-director operation, a load factor instruction made available to the pilot on the PFD screen.
It is apparent that, during automatic-pilot operation, flight control systems with large loops delivering load factor commands are more reactive than those with large loops delivering longitudinal attitude angle deviation commands, so that it would be beneficial to adapt them to conventional flight controls were it not for the pilot's difficulty in following a load factor instruction with conventional flight controls that are more appropriate to the following of longitudinal attitude angle instructions.